VEGA VEGAPULS 56 Profibus PA - Manual (Page 16)

16 VEGAPULS 56 Profibus PA
Types and versions
2.3 Bus configuration
The type of radar sensor you use depends
on your process requirements and mounting
conditions, as well as on the requirements of
your control, regulative, or process control
system.
VEGAPULS 56 Profibus radar sensors are
sensors for use in Profibus PA environment.
Profile 3 has been implemented in the sen-
sors. A measuring system consists of one or
several sensors, one or several segment
couplers and one DP master computer, such
as e.g. a S7 PLC with Profibus interface or a
process control system with Profibus DP-
Master-Slot. The processing unit, e.g. the
PLC, evaluates the level-proportional, digital
measuring signals in a number of evaluation
routines and puts them to use process-spe-
cifically.
On the following four pages you will see
schematic illustrations of the bus configura-
tion.
The automation system as Master-Class 1
takes over bus control completely. It reads
out all signals cyclically and, if necessary,
gives instructions to the participants (e.g.
sensors). Besides this, additional master
systems (e.g. visualisation systems or ad-
justment tools) can be connected to the DP
bus. These systems operate as so-called
Master-Class 2 participants. Like the Master-
Class 1 system, they can read out signals,
give instructions and operate in the acyclical
mode.
A DP bus does not allow power supply via
signal cable, whereas the PA bus does. Both,
DP and PA, require at a minimum a screened
two-wire cable. The DP bus can additionally
have up to 8 cores (screened), of which
some can be supply cables (see also "Instal-
lation Guides PA + DP“ of the Profibus User
Organisation (PNO).
Each participant on the bus must have an
unambiguous address. The addressing
covers both bus levels. A Profibus DP net-
work can have max. 126 participants, includ-
ing all participants on the PA level. In
practice, each Master-Class 1 computer gets
address 1 and the Master-Class 2 comput-
ers address 10 … 20. As a rule, the slaves or
participants get the addresses 21 … 126. On
the Profibus PA network segment, max. 32
sensors are possible on one PA segment
coupler.
Ex environment
In Ex environment, intrinsically safe (EEx ia)
PA sensors are used with Ex segment cou-
plers. Generally, the number of PA sensors
on a segment coupler (Ex or non Ex) de-
pends on the current requirement of the
sensors and on the current supplied by the
segment coupler. Segment couplers for
EEx ia environment provide 90 … 110 mA.
The number of sensors results from the sum
of:
- the basic current intake of all sensors
- plus 9 mA communication signal
- plus the leakage currents of all sensors
- plus a recommended current reserve
(approx. 10 mA)
The min. basic current has been set at 10 mA
according to the Profibus specification.
VEGA Profibus sensors constantly consume
a basic current of 10 mA and operate without
leakage current requirement, so that in Ex
environment, up to max. ten VEGA sensors
can be operated on one segment coupler.